The calcium that triggers muscle contraction is stored in synaptic vesicles. the motor neuron. the mitochondria. the endoplasmic reticulum.

The calcium that triggers muscle contraction is stored in synaptic vesicles called endoplastic reticulum. that is not the only function of  the endoplastic reticulum it is also a network of a membranous tubules within the cytoplasm of a eucryotic cell continuous with the nuclear membrane.